1. Field of the Invention
The present invention generally refers to the field of thermal energy storage, which may be abbreviated as TES, and more particularly to the storage of such energy by means of phase change materials, often referred to hereinafter as PCM's. By PCM's I refer to certain chemical compositions that store heat energy in one phase, which energy is then either released in the form of heat or which acquire heat from an ambient fluid as the PCM changes phase.
2. The Prior Art
The use of certain chemical compositions for "cool" thermal energy storage is an art that has yet to achieve its full market potential, although there are at present a plurality of such "cool" storage installation in successful commercial operation in this country. The use of PCM's to store coolness, i.e., to freeze at a predetermined temperature, usually below about 50.degree. F., has successfully been carried out, particularly where time-of-use rates make such applications commercially practical. Commercial feasibility occurs where the differential between peak and off-peak electric energy rates, as well as the application of a so-called demand charge for the use of electricity during peak periods, makes the installation of a TES system viable, e.g., the cost of the system, as measured against the savings, provides a pay back period of perhaps three years.
Using a cool storage TES system with a PCM having a melting/freezing point plateau of somewhat below 50.degree. F., the charging cycle is automatically timed to take place during off-peak hours. By the provision of suitable piping between a chiller and a tank holding containerized PCM's, cold water from the chiller is circulated to the tank, into contact with the containers of PCM's, and then circulated from the tank back to the chiller for recooling. This procedure continues until the PCM's have been frozen, i.e., until they have changed their state from liquid to solid, thereby transferring their heat of fusion to the cold water that has chilled them, or another cold fluid used for that purpose. That is the charging cycle.
In the discharge cycle, line water at a temperature in excess of the melting point of the PCM is circulated through the tank of frozen, containerized composition, and thereby chilled approximately to the temperature of the frozen PCM's which, in a manner of speaking, may be said to have stored coolness. As the relatively warm water or other fluid is passed into thermal contact with the frozen PCM's, the latter melt, and in so doing use the heat energy of the relatively warm water to satisfy their heat of fusion, thereby lowering the temperature of the water, which is then circulated to the water-to-air heat exchangers in the structure to be cooled. This procedure continues until substantially all of the coolness stored in the frozen PCM's has been exhausted by the melting of the PCM's and, if desirable, by the use of the specific heat differential between the PCM's in liquid form and the warmer water.
By properly sizing the TES unit, with knowledge of the extent of the peak period the TES system can supply all of the cooling requirements of a building for the entirety of the on-peak period, thereby making it unnecessary to use the high electricity consumption chiller during on-peak periods. Alternatively, the TES system can supply only a part of the on-peak load, which would permit the use of a smaller chiller. In either mode of use, a PCM thermal energy storage system can effect substantial savings to the owner and provide a desirable leveling effect to the 24-hour load profile of an electric utility during summer months when electricity use for air conditioning is maximal.
Any PCM utilized for thermal storage will have generic requirements. These include maintenance of a near constant temperature throughout the phase change cycle, also referred to as a melting/freezing point plateau, a relatively high latent heat of fusion, and a relatively high density. Such a PCM will be able to store large quantities of coolness, so as to make its use practical in commercial installations. Of course, the PCM must maintain its desirable thermal energy storage capacity over thousands of freeze-thaw cycles, indeed almost indefinitely, and must be abundantly available and relatively inexpensive. Such compositions are available and now in use.
From a commercial point of view, the composition that has thus far found greatest application as a PCM is a eutectoid salt composition based on sodium sulfate decahydrate. This salt, like most materials, including water, has a tendency to supercool; however, many years ago Dr. Maria Telkes, a pioneer in the field, discovered that sodium tetraborate would ameliorate the problem of supercooling of this type of PCM. Unfortunately, the lowest temperature at which a sodium sulfate decahydrate eutectoid salt mixture has been found to freeze is 47-48.degree. F. As a consequence, in some situations where a full storage system design is employed or for other reasons, and it is requisite that chilled water be supplied from the PCM tank at less than 47.degree. F., a sodium sulfate decahydrate PCM is not appropriate.
More recently, it has been learned that a specific NaOH/H.sub.2 O solution can be employed as a PCM for cool storage. The applicant has become aware that an aqueous solution of about 46-47% NaOH in water has a freezing/melting point plateau at approximately 41.degree. F., a highly propitious temperature for a PCM. At this temperature the water solution of NaOH melts congruently, i.e., when it melts, it exists as a stable solid in equilibrium with a liquid of the same composition. With PCM's that do not melt congruently, e.g., sodium sulfate-based eutectoid salts, a thickening or gelling agent may be used to maintain this stable solid/liquid equilibrium. Since at that particular concentration, rather than as some other, random concentration, the NaOH/H2O solution has a melting/freezing point plateau, it is exceptionally well suited for employment as a PCM.
It will be apparent that at a random freezing/melting point on the liquidus line of a phase diagram of a NaOH/H.sub.2 O binary system, freezing or melting will only begin. As, due to such partial freezing or melting, the concentration of the sodium hydroxide changes, so does the freezing/melting point, and there is no freezing/melting point plateau. In a sodium hydroxide-water system or, indeed, any system that will be adapted for employment as a PCM, it is requisite that there be a substantial plateau at the freezing/melting point; otherwise, on melting, for example, a varying melting point would cause the fluid flowing from the storage tank to do so at varying temperatures, making that fluid unsuitable for use in air conditioning a building because of its non-constant exit temperatures.
With all the benefits of a 41.degree. F. melting/freezing point NaOH in water solution, one serious drawback to its use is that significant supercooling tends to occur. In the laboratory this can be remedied by seeding, agitation, etc.; in a commercial installation where many thousands of containers of the PCM are stored in a tank and are not subject to easy access, the problem cannot be easily rectified. Experimentation has confirmed that the means for overcoming supercooling in eutectoid salt compositions--sodium tetraborate decahydrate--is ineffective in performing that function in a 41.degree. F. melting point NaOH/H2O solution.
As a consequence, it is a primary object of the present invention to provide a supercooling inhibitor for a 47-48% NaOH/water solution so that when the temperature of such solution is lowered below the 41.degree. F. level, crystallization of the solution is initiated without additional seeding or agitation.
It is another object of this invention to provide such a supercooling inhibitor, which may also be termed a nucleating agent, which will be economically effective, abundantly available, and which will perform its function in small quantities relative to the remainder of the NaOH/H.sub.2 solution, since whatever percentage of the total PCM is occupied by the nucleating agent, that percent is a part of the entire solution that does not per se freeze and, therefore, does not itself store cool energy by its heat of fusion.